The present invention is directed to programmable logic controller (PLC) systems which control a wide variety of manufacturing processes. A typical configuration is shown in FIG. 1, and comprises a power supply 11 which supplies power to a processor 13 which is coupled to an address bus 21, a data bus 23 and control bus 25, each of which are coupled to individual controller slots 15. As shown in FIG. 1, a single processor typically accesses up to 16 controller cards, each of which is plugged into one of 16 controller slots 15. The controller cards themselves interface, for example, external apparatus such as robots on an assembly line through input cards for receiving signals from the robots and output cards for generating signals which control the operation of the robots through a feedback control loop.